The invention relates generally to wire bonds for connecting integrated circuits to computer circuit boards. In particular, the invention relates to producing a wire connection having improved pull-test characteristics over conventional techniques. Approximately ninety percent of all semiconductor packages are manufactured using conventional wire bonding techniques.
Conventional wire bonding techniques, e.g., thermosonic ball bonding, employ a capillary dispenser to release a conductive wire, typically gold (Au). Ultrapure gold, although extremely resistant to corrosion, is relatively soft and fragile. Hence, trace amounts of impurities such as 3 ppm to 10 ppm of beryllium (Be), 0.1 ppm to 100 ppm of copper (Cu), 1 ppm to 5 ppm of iron (Fe), and/or 3 ppm to 10 ppm of silver (Ag) are added to create interstitial defects within the molecular structure of the gold wire to improve overall ductility and toughness.
The process generally begins with the creation of a free air ball (FAB) extended beneath the capillary dispenser by using an electronic flame off (EFO) system to melt a small portion of the wire. The surface tension of the molten metal forms a spherical shape (or ball) as the metal cools and solidifies. The bonding surface is heated using a specially designed temperature-controlled wire bonding fixture. Upon creation of the FAB, a first bond can be formed by depositing gold from the capillary dispenser onto a die pad on an integrated circuit (IC) chip die, while employing sufficient ultrasonic power (in watts, typically 0.5 W to 2.0 W), ultrasonic frequency (typically 50 kHz to 75 kHz), temperature (typically 100° C. to 200° C.), machine-over-travel (typically 0.001 inch to 0.005 inch), and duration (in milliseconds, e.g., 50 ms to 150 ms) to cause the plastic deformation of the wire and intermetallic connection of the wire to the bonding surface. This ensures electrical and mechanical contacts between the two metal surfaces without solder.
The capillary dispenser is then raised and repositioned over the second bond site on the circuit board pad, which creates a precisely shaped wire connection called a wire spool. The second loop is then created by depositing the gold onto the circuit board pad via the capillary dispenser, while employing conditions of force, temperature and duration to induce plastic deformation of the wire and intermetallic connection of the wire to the bonding surface. This ensures electrical and mechanical contacts between the two metal surfaces.